Readily polymerizable acrylate monomers are important chemicals of commerce. The current production of methyl methacrylate and related monomers is approximately 1.3 billion pounds per year. These monomers are most often prepared by methods involving a distillation step at elevated temperature and/or reduced pressure.
Acrylic acid and derivatives thereof are currently preferably prepared by the oxidation of propylene. The process involves a two-step oxidation, followed by an extraction, and vacuum distillation. Acrylic acid can also be produced by the reaction of acetylene, carbon monoxide, and water in the presence of nickel carbonyl. This method also involves the removal of product by distillation. A similar process which additionally includes methyl or ethyl alcohol as a reactant is employed for the preparation of methyl or ethyl acrylate, respectively. Again, distillation is employed to purify and collect the product.
A widely employed process for the preparation of methacrylic acid involves the acid hydrolysis of acetone cyanohydrin. Methyl methacrylate is prepared by a similar process involving the acid methanolysis of acetone cyanohydrin. Each of these methods likewise employs vacuum distillation at elevated temperatures (e.g., about 100.degree. C.) in order to recover the product.
In the above-described processes for the preparation of acrylate monomers, like all other preparations of polymerizable monomers, care must be exercised to remove the products from the reaction mixture and to inhibit the monomer before uncontrolled polymerization can ensue. The reactivity of these acrylate monomers necessitates the use of a relatively large amount of process inhibitor in order to prevent polymerization. Currently used process inhibitors include hydroquinone, the methyl ether of hydroquinone, and p-benzoquinone. A compound which has been proposed for use as a process inhibitor is n-nitroso-phenylhydroxylamine. However, this compound is expensive and is reported to be extremely toxic.
When investigating the usefulness of a compound as a process inhibitor for acrylate monomer, a number of characteristics are evaluated. The compound must exhibit an effectiveness as a polymerization inhibitor equal to or greater than the currently employed or proposed process inhibitors. The compounds should exhibit a lower environmental impact and toxicity than currently employed and proposed process inhibitors. A process inhibitor which is proposed for use in acrylate monomer systems should be soluble in acrylate monomers, such as methyl methacrylate, or in suitable carrier solvents at levels which are high enough to allow the preparation of commercially useful stock solutions. Also, the process inhibitor must be a compound which does not entrain with the acrylate monomer during the final purification of the monomer.
It has now been found that all of these characteristics are surprisingly and unexpectedly exhibited by phenyl-para-benzoquinone, diphenyl-para-benzoquinone, and mixtures thereof.
Japanese Kokai Tokkyo Koho No. 81 86,123 discloses the use of 2-phenyl-1,4-benzoquinone as a polymerization inhibitor for aromatic vinyl compounds. The use of 2,5-diphenyl-p-benzoquinone as a polymerization inhibitor for a polyester resin system diluted with styrene is disclosed in Proceedings of the 23rd Annual Technical Conference of the Reinforced Plastics/Composites Division of the Society of the Plastics Industry (Plastics Ind., Inc.: New York) 1968. The use of diphenyl-p-benzoquinone in admixture with another inhibitor to inhibit the crosslinking of polyolefins is disclosed in British Patent Specification No. 1,077,634. The use of diphenyl-p-benzoquinone as an inhibitor in the curing of unsaturated polyester is disclosed in U.S. Pat. No. 3,026,286. None of these prior art references recognizes the unique characteristics of phenyl-para-benzoquinone and/or diphenyl-para-benzoquinone which make these compounds, either alone or in mixtures, desirable as process inhibitors for acrylate monomers.